Proposed systems for providing digital audio broadcasting in the FM radio band are expected to provide near CD-quality audio, data services and more robust coverage than existing analog FM transmissions. However, until such time as a transition to all-digital DAB can be achieved, many broadcasters require an intermediate solution in which the analog and digital signals can be transmitted simultaneously within the same licensed band. Such systems are typically referred to as hybrid in-band on-channel (HIBOC) DAB systems, and are being developed for both the FM and AM radio bands.
In order to prevent significant distortion in conventional analog FM receivers, the digital signal in a typical FM HIBOC DAB system is, for example, transmitted in two side bands, one on either side of the analog FM host signal, using orthogonal frequency division multiplexing (OFDM) sub-carriers. In an OFDM communication system, the digital signal is modulated to a plurality of small sub-carrier frequencies that are then transmitted in parallel.
In the United States, the frequency plan established by current FCC regulations separates each transmitting station in a geographical area by 800 KHz. Any transmitting stations in adjacent geographical areas, however, are separated from a local transmitting stations by only 200 KHz. Thus, a particularly significant source of interference in such a system is known as first adjacent analog FM interference. This interference results when a portion of a FM host carrier in an adjacent geographic area overlaps in frequency with a portion of a digital signal side band. Although first adjacent analog FM interference, when present, typically affects only one of the two digital side bands, it nonetheless represents a limiting factor on the performance of DAB systems. The presence of a strong first adjacent interference signal will significantly degrade the performance of the digital signal transmissions, even when one of the two side bands is free from interference.
An FM channel suffers from dispersion in both the time and frequency domains. In the time domain, severe multiple path propagation can yield a delay spread ranging from 3 to 30 microseconds in urban and suburban environments. Such a delay spread range corresponds to a channel coherence frequency range of 30 to 300 kHz, which can be comparable to the signal bandwidth at the upper limit and causes flat fades for channels exhibiting low delay spread, such as in a dense urban environment. In the frequency domain, a slow moving vehicle may experience a frequency dispersion of 0.2 Hz while a fast moving vehicle may experience a frequency dispersion of 15 Hz. For static channels, such as a slow moving vehicle, the channel varies slowly in time and as such, time diversity schemes alone cannot combat various channel impairments such as selective and flat fading conditions.
A need therefore exists for a multi-stream transmission system that exhibits both time and frequency diversity. A further need exists for a method and apparatus that diversifies a PAC signal across both the time and frequency domains within the allocated bandwidth and time delay.